The invention relates to methods and to apparatuses for producing annular parts of composite material and preforms for such parts, and also to the parts and to the preforms themselves. More particularly, the invention relates to methods and apparatus for producing preforms by winding a fibrous strip on a mandrel, the wound layer assembly being intended to be cut into rings either before or after densification by a matrix.
Conventional methods of producing preforms for composite material parts consist in stacking flat layers of fibrous material, cutting or machining the assembly to obtain a preform with the desired shape, and then densifying the preform. When the part to be produced is a brake disk or some other annular part, about half of the weight of the material is lost when producing annular preforms from an assembly of stacked layers.
A number of proposals have been put forward to reduce such waste. One proposal consists in assembling an annular preform to be densified from layers of fibrous material each in the form of juxtaposed sectors, the layers then being stacked. Such a method reduces waste but does not avoid it.
A further proposal made in French patent application FR-A-2 506 672 is described below with reference to FIGS. 1A to 1D. Annular or cylindrical elements are produced by winding a fibrous strip on a cylindrical mandrel (FIG. 1A) to produce a cylindrical sleeve (FIG. 1B). During winding, the superposed layers are connected together by needling. The cylindrical sleeve can be cut perpendicularly to its axis to obtain annular preforms to be densified (FIG. 1C).
A method similar to the above has also been described in French patent application FR-A-2 584 107.
That method avoids wasting material, but the parts made from preforms produced by that method have disadvantageous features which come to light during service. Brake disks are subjected to shear stresses in a tangential direction during use. The stresses are particularly high in the notches formed in the inner or outer border (FIG. 1D) to connect the disk with a moving or a fixed portion. Such shear stresses E can cause the part to delaminate, i.e., it is destroyed by the layers in the preform separating.